1. Field of the Invention
This invention relates to gas turbine engine exhaust nozzles and, more particularly, propulsion systems of the thrust vectorable variety.
2. Description of the Prior Art
The high velocity imparted to exhaust gases of a gas turbine by an exhaust nozzle provides thrust for propulsion. This thrust is substantially opposite to the direction of flow of exhaust gases exiting the nozzle. Consequently, if direction of the exhaust gases is changed, the direction of propulsive thrust is correspondingly varied.
Typically, aircraft gas turbine engines are provided with nozzles which are fixed in the axial direction, and aircraft maneuvering and lift are accomplished solely by airframe control surfaces. Advanced aircraft configurations contemplate, and may even require, the selective redirection (or vectoring) of gas turbine engine thrust in order to enhance aircraft performance and to provide the aircraft with operational characteristics heretofore deemed impractical. For example, if the exhaust of a conventionally installed aircraft turbine engine is directed downwardly, rather than rearwardly, to a direction substantially perpendicular to the engine longitudinal axis, the resulting upward thrust would provide direct lift for the aircraft and, if properly controlled, a vertical takeoff or short takeoff and landing capability. Similarly, thrust vectoring during flight can greatly increase aircraft maneuverability since the thrust force can augment the maneuvering forces of the aircraft control surfaces.
In order to accomplish such thrust vectoring, a device is required to efficiently and practically alter the direction of gas turbine engine exhaust nozzle gases. Many devices which are well known to those skilled in the art have been developed for the purpose of accomplishing thrust vectoring. Among these devices is an exhaust nozzle employing an upper and a lower flap that are angled simultaneously for the purpose of deflecting exhaust gas in an upward or downward direction. Increasing the angle of the flaps increases the amount of turning that is imparted to the exhaust gas flow. In practice, the flaps are tilted through a relatively small angle, on the order of approximately 20 degrees, for the purpose of the inflight maneuvering, and are tilted to a greater angle, on the order of 40-70 degrees, for the purpose of short takeoff and landing (STOL) applications. While this prior art system has provided a thrust-vectorable nozzle, problems have been encountered in respect to fluid flow within the nozzle because of the location of a minimum flowpath area or throat of the nozzle. In the prior art device, the throat has been located within a fixed duct portion of the nozzle and this is the region where the exhaust gases accelerate from subsonic to supersonic velocities. The upper and lower flaps employed for vectoring of the thrust have been located downstream of the throat area of the nozzle causing the exhaust gases to be turned after they have attained supersonic velocities. This has proved to be an inefficient mode for vectoring of exhaust and, in some configurations, has even created a dual throat nozzle creating conditions of excessive shock sometimes causing harm to nozzle components. While some prior art nozzles have been developed that turn the exhaust gas flow prior to supersonic acceleration, these nozzles generally are either incapable of sufficiently vectoring the exhaust flow to permit STOL operations or have been excessively heavy and cumbersome, particularly for applications in a high performance jet fighter.